Debra
04-27-2007, 04:36 PM
Dear Mr. Lee,
I have read much of your columns and answers over the years and still enjoy learning more about model engines (and refreshing my memory as well!).
Today at the flying field, I was watching a club member tune the engine of his Bobcat powered by a pusher glow engine. What caught my attention is that he was raising the nose up while adjusting the top end - this being acceptable practice for regular tractor configuration. My question to you is shouldn't he be lowering the nose to check the top end to avoid a lean run? The argument I heard at the club was that the nose is pointing up in the take off configuration and that you normally do not dive at full throttle.
What is your opinion?
Thanks,
B-
Dear B,
Holding the aircraft's nose up when adjusting the mixture is simulating the in-flight attitude of the aircraft during take off and climbing maneuvers when the fuel is being drawn a longer distance causing a leaning condition. However, with a pusher aircraft the fuel tank is ahead of the engine and you have an entirely different set of circumstances to contend with. During take off and when climbing the fuel tank is higher than the engine causing a richening tendency along with the acceleration force during take off. In a dive, the tank is lower than the engine, but, the fuel remains in the back of the tank due to acceleration. As such, the engine does not have to draw fuel as far as it does with a tractor mounted engine.
When you saw the club member adjust his engine with the tail down he was probably dOlng so to compensate for the richening tendency on take off and in climbing maneuvers.
Regards
Clarence Lee
I have read much of your columns and answers over the years and still enjoy learning more about model engines (and refreshing my memory as well!).
Today at the flying field, I was watching a club member tune the engine of his Bobcat powered by a pusher glow engine. What caught my attention is that he was raising the nose up while adjusting the top end - this being acceptable practice for regular tractor configuration. My question to you is shouldn't he be lowering the nose to check the top end to avoid a lean run? The argument I heard at the club was that the nose is pointing up in the take off configuration and that you normally do not dive at full throttle.
What is your opinion?
Thanks,
B-
Dear B,
Holding the aircraft's nose up when adjusting the mixture is simulating the in-flight attitude of the aircraft during take off and climbing maneuvers when the fuel is being drawn a longer distance causing a leaning condition. However, with a pusher aircraft the fuel tank is ahead of the engine and you have an entirely different set of circumstances to contend with. During take off and when climbing the fuel tank is higher than the engine causing a richening tendency along with the acceleration force during take off. In a dive, the tank is lower than the engine, but, the fuel remains in the back of the tank due to acceleration. As such, the engine does not have to draw fuel as far as it does with a tractor mounted engine.
When you saw the club member adjust his engine with the tail down he was probably dOlng so to compensate for the richening tendency on take off and in climbing maneuvers.
Regards
Clarence Lee